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Muscle Tissue

Muscle Tissue. Alternating contraction and relaxation of cells Chemical energy changed into mechanical energy. Properties of Muscle Tissue. Excitability respond to chemicals released from nerve cells Contractility ability to shorten and generate force Extensibility

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Muscle Tissue

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  1. Muscle Tissue • Alternating contraction and relaxation of cells • Chemical energy changed into mechanical energy Shina ALAGIA 2005

  2. Properties of Muscle Tissue • Excitability • respond to chemicals released from nerve cells • Contractility • ability to shorten and generate force • Extensibility • ability to be stretched without damaging the tissue • Elasticity • ability to return to original shape after being stretched Shina ALAGIA 2005

  3. 3 Types of Muscle Tissue • Skeletal muscle • attaches to bone, skin or fascia • striated with light & dark bands visible with scope • voluntary control of contraction & relaxation Shina ALAGIA 2005

  4. 3 Types of Muscle Tissue • Cardiac muscle • striated in appearance • involuntary control • autorhythmic because of built in pacemaker Shina ALAGIA 2005

  5. 3 Types of Muscle Tissue • Smooth muscle • attached to hair follicles in skin • in walls of hollow organs • nonstriated in appearance • involuntary Shina ALAGIA 2005

  6. Functions of Muscle Tissue • Producing body movements • Stabilizing body positions • Movement of substances within the body • blood, lymph, urine, air, food and fluids, sperm • Producing heat contractions of skeletal muscle Shina ALAGIA 2005

  7. Skeletal Muscle -- Connective Tissue • Superficial fascia is loose connective tissue & fat underlying the skin • Deep fascia = dense irregular connective tissue around muscle • Connective tissue components of the muscle include • epimysium = surrounds the whole muscle • perimysium = surrounds bundles (fascicles) of 10-100 muscle cells • endomysium = separates individual muscle cells • All these connective tissue layers extend beyond the muscle belly to form the tendon Shina ALAGIA 2005

  8. Connective Tissue Components Shina ALAGIA 2005

  9. Nerve and Blood Supply • Each skeletal muscle is supplied by a nerve, artery and two veins. • Each motor neuron supplies multiple muscle cells (neuromuscular junction) • Each muscle cell (fiber) is supplied by one motor neuron terminal branch and is in contact with one or two capillaries. • nerve fibers & capillaries are found in the endomysium between individual cells Shina ALAGIA 2005

  10. Muscle Fiber or Myofibers • Muscle cells(fibers) are long, cylindrical & multinucleated • Sarcolemma = muscle cell membrane • Sarcoplasm (cytoplasm) filled with tiny threads called myofibrils & myoglobin (red-colored, oxygen-binding protein) Shina ALAGIA 2005

  11. Myofibrils & Myofilaments • Muscle fibers are filled with threads called myofibrils. • Myofibrils are further made of filaments (thick & thin filaments) that are contractile proteins of muscle Shina ALAGIA 2005

  12. Filaments and the Sarcomere • Thick and thin filaments overlap each other in a pattern that creates striations (light I bands and dark A bands) • The I band region contains only thin filaments. • They are arranged in compartments called sarcomeres, separated by Z discs/lines. • In the overlap region, six thin filaments surround each thick filament Shina ALAGIA 2005

  13. Thick & Thin Myofilaments • Supporting proteins (M line, titin and Z disc help anchor the thick and thin filaments in place) Shina ALAGIA 2005

  14. Overlap of Thick & Thin Myofilaments within a Myofibril Dark(A) & light(I) bands visible with an electron microscope Shina ALAGIA 2005

  15. The Proteins of Muscle • Myofibrils are built of 3 kinds of protein • contractile proteins • myosin and actin • regulatory proteins which turn contraction on & off • troponin and tropomyosin • structural proteins which provide proper alignment, elasticity and extensibility • titin, myomesin, nebulin and dystrophin Shina ALAGIA 2005

  16. The Proteins of Muscle -- Myosin • Thick filaments are composed of myosin • each molecule resembles two golf clubs twisted together • myosin heads (cross bridges) extend toward the thin filaments • Held in place by the M line proteins. Shina ALAGIA 2005

  17. The Proteins of Muscle -- Actin • Thin filaments are made of actin, troponin, & tropomyosin • The myosin-binding site on each actin molecule is covered by tropomyosin in relaxed muscle • The thin filaments are held in place by Z lines. From one Z line to the next is a sarcomere. Shina ALAGIA 2005

  18. Transverse Tubules • T (transverse) tubules are invaginations of the sarcolemma into the center of the cell • carry muscle action potentials down into cell • Mitochondria lie in rows throughout the cell Shina ALAGIA 2005

  19. Sarcoplasmic Reticulum (SR) • System of tubular sacs similar to smooth ER • Located around myofibrils • TRIAD= T tubule + terminal cisterns/lateral sacs. Lateral sacs store Ca+2 in a relaxed muscle Shina ALAGIA 2005

  20. Atrophy and Hypertrophy • Atrophy • wasting away of muscles • caused by disuse (disuse atrophy) or severing of the nerve supply (denervation atrophy), diet • the transition to connective tissue can not be reversed • Hypertrophy • increase in the diameter of muscle fibers • resulting from very forceful, repetitive muscular activity and an increase in myofibrils, SR & mitochondria Shina ALAGIA 2005

  21. Neuromuscular Junction (NMJ) or Synapse • NMJ = Neuro-muscular junction • end of neuron nears the surface of a muscle fiber (remain separated by gap) Shina ALAGIA 2005

  22. Structures of NMJ Region • Synaptic end bulbs are swellings of axon terminals • End bulbs contain synaptic vesicles filled with acetylcholine (ACh) • Motor end plate on muscle membrane contains 30 million ACh receptors. Shina ALAGIA 2005

  23. Pharmacology of the NMJ: FYI • Botulinum toxin blocks release of neurotransmitter at the NMJ so muscle contraction can not occur • bacteria found in improperly canned food • death occurs from paralysis of the diaphragm • Curare (plant poison from poison arrows) • causes muscle paralysis by blocking the ACh receptors • used to relax muscle during surgery • Neostigmine (anticholinesterase agent) • blocks removal of ACh from receptors so strengthens weak muscle contractions of myasthenia gravis • also an antidote for curare after surgery is finished Shina ALAGIA 2005

  24. Sliding Filament Mechanism Of Contraction • Myosin headspull on thin filaments • Thin filaments slide inward • Z Discs come toward each other • Sarcomeres shorten.The muscle fiber shortens. The muscle shortens • Notice :Thick & thin filaments do not change in length Shina ALAGIA 2005

  25. How Does Contraction Begin? • Nerve impulse reaches a neuron terminal & synaptic vesicles release acetylcholine (ACh) • ACh diffuses to receptors on the motor end plate • A muscle action potential (membrane potential change) spreads over sarcolemma and down into the transverse tubules • SR/TRIAD releases Ca+2 into the sarcoplasm • This is Excitation Shina ALAGIA 2005

  26. Excitation - Contraction Coupling • All the steps that occur from the muscle action potential reaching the T tubule to contraction of the muscle fiber. Shina ALAGIA 2005

  27. Contraction Cycle • Repeating sequence of events, in response to excitation that cause the thick & thin filaments to move past each other. • 4 steps to contraction cycle • ATP hydrolysis • attachment of myosin head to actin (cross bridge attachment) • power stroke (and ADP dropped) • detachment of myosin from actin • Cycle keeps repeating as long as there is ATP available & high Ca+2 level near thin filament Shina ALAGIA 2005

  28. Steps in the Contraction Cycle • Notice how the myosin head attaches and pulls on the thin filament with the energy released from ATP Shina ALAGIA 2005

  29. ATP and Myosin • Myosin heads are activated by ATP • Activated heads attach to actin & pull (power stroke) • ADP is released. (ATP releases P & ADP & energy) • Thin filaments slide past the thick filaments • New ATP binds to myosin head & detaches it from actin • All of these steps repeat over and over • if ATP is available & • Ca+ level near the troponin-tropomyosin complex is high Shina ALAGIA 2005

  30. Relaxation • Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft • Muscle action potential ceases • Active transport pumps Ca2+ back into storage in the lateral sacs • FYI: Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR (Ca+2 concentration 10,000 times higher than in cytosol) • Tropomyosin-troponin complex recovers binding site on the actin Shina ALAGIA 2005

  31. Rigor Mortis • Rigor mortis is a state of muscular rigidity that begins 3-4 hours after death and lasts about 24 hours • After death, Ca+2 ions leak out of the SR and allow myosin heads to bind to actin • Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells. Shina ALAGIA 2005

  32. Length of Muscle Fibers • Optimal overlap of thick & thin filaments • produces greatest number of crossbridges and the greatest amount of tension • Overstretched muscle (past optimal length) • fewer cross bridges exist & less force is produced • Overly shortened muscle (less than optimal) • fewer cross bridges exist & less force is produced Shina ALAGIA 2005

  33. The Motor Unit • Motor unit = one motor neuron & all the skeletal muscle fibers it stimulates • One nerve cell supplies on average 150 muscle fibers that all contract in unison. • Total strength of a muscle contraction depends on how many motor units are activated & how large the motor units are Shina ALAGIA 2005

  34. Twitch Contraction • Brief contraction of all fibers in a motor unit • single action potential in its motor neuron • Myogram = graph of a twitch contraction • the action potential lasts 1-2 msec • the twitch contraction lasts from 20 to 200 msec Shina ALAGIA 2005

  35. Parts of a Twitch Contraction • Contraction Period • 10 to 100 msec • filaments slide past each other • Relaxation Period • 10 to 100 msec • active transport of Ca+2 into SR • Refractory Period • muscle can not respond (sarcolemma) Shina ALAGIA 2005

  36. Wave Summation • If second stimulation applied after the refractory period but before complete muscle relaxation---second contraction is stronger than first Shina ALAGIA 2005

  37. Complete and Incomplete Tetanus • Unfused/incomplete tetanus • only partial relaxation between stimuli • Fused/complete tetanus • a sustained contraction with no relaxation between stimuli Shina ALAGIA 2005

  38. Explanation of Summation & Tetanus • Wave summation & both types of tetanus result from Ca+2 remaining in the sarcoplasm • Force of 2nd contraction is easily added to the first. Shina ALAGIA 2005

  39. Motor Unit Recruitment • Motor units in a whole muscle fire asynchronously • some fibers are active others are relaxed • delays muscle fatigue so contraction can be sustained • Produces smooth muscular contraction • not series of jerky movements • Precise movements require smaller contractions • motor units must be smaller (less fibers/nerve) • Large motor units are active when large tension is needed Shina ALAGIA 2005

  40. Muscle Tone • Involuntary contraction of a small number of motor units (alternately active and inactive in a constantly shifting pattern) • keeps muscles firm even though relaxed • does not produce movement • Essential for maintaining posture (head upright) • Important in maintaining blood pressure • tone of smooth muscles in walls of blood vessels Shina ALAGIA 2005

  41. Isotonic and Isometric Contraction • Isotonic contractions = a load is moved • Isometric contraction = no movement occurs • tension is generated without muscle shortening • maintaining posture & supports objects in a fixed position Shina ALAGIA 2005

  42. Muscle MetabolismProduction of ATP in Muscle Fibers • Muscle uses ATP at a great rate when active • Sarcoplasmic ATP only lasts for few seconds • 3 sources of ATP production within muscle • creatine phosphate • anaerobic cellular respiration • aerobic cellular respiration Shina ALAGIA 2005

  43. Creatine Phosphate • Excess ATP within resting muscle used to form creatine phosphate • Creatine phosphate 3-6 times more plentiful than ATP within muscle • Its quick breakdownprovides energy for creation of ATP • Sustains maximal contraction for 15 sec (used for 100 meter dash). • Athletes tried creatine supplementation • gain muscle mass but shut down bodies own synthesis Shina ALAGIA 2005

  44. Anaerobic Cellular Respiration • ATP produced from glucose breakdown into pyruvic acid during glycolysis • if no O2 present • pyruvic converted to lactic acid which diffuses into the blood • Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity (200 meter race) Shina ALAGIA 2005

  45. Aerobic Cellular Respiration • ATP for any activity lasting over 30 seconds • if sufficient oxygen is available, pyruvic acid enters the mitochondria to generate ATP, water and heat • fatty acids and amino acids can also be used by the mitochondria • Provides 90% of ATP energy if activity lasts more than 10 minutes Shina ALAGIA 2005

  46. Muscle Fatigue • Inability to contract after prolonged activity • central fatigue is feeling of tiredness and a desire to stop (protective mechanism) • depletion of creatine phosphate • Factors that contribute to muscle fatigue • insufficient oxygen or glycogen • buildup of lactic acid and ADP • insufficient release of acetylcholine from motor neurons Shina ALAGIA 2005

  47. Classification of Muscle Fibers • Slow oxidative (slow-twitch) • red in color (lots of mitochondria, myoglobin & blood vessels) • prolonged, sustained contractions for maintaining posture • Fast oxidative-glycolytic (fast-twitch A) • red in color (lots of mitochondria, myoglobin & blood vessels) • split ATP at very fast rate; used for walking and sprinting • Fast glycolytic (fast-twitch B) • white in color (few mitochondria & BV, low myoglobin) • anaerobic movements for short duration; used for weight-lifting Shina ALAGIA 2005

  48. Fiber Types within a Whole Muscle • Most muscles contain a mixture of all three fiber types • Proportions vary with the usual action of the muscle • neck, back and leg muscles have a higher proportion of postural, slow oxidative fibers • shoulder and arm muscles have a higher proportion of fast glycolytic fibers • All fibers of any one motor unit are same. Shina ALAGIA 2005

  49. FYI SLIDES Shina ALAGIA 2005

  50. Anabolic Steroids • Similar to testosterone • Increases muscle size, strength, and endurance • Many very serious side effects • liver cancer • kidney damage • heart disease • mood swings • facial hair & voice deepening in females • atrophy of testicles & baldness in males Shina ALAGIA 2005

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